1. Field of the Invention
The present invention relates to a distributed feedback (DFB) type semiconductor laser and, more particularly, to a ridge type semiconductor laser of laterally-coupled distributed feedback (LC-DFB).
2. Description of the Related Art
The DFB semiconductor lasers are known as a semiconductor device which has a periodic structure i.e., grating in one direction in which induced laser beam propagate to improve the coupling of forward and backward waves to utilize for wave-selectivity in the optical feedback. The DFB semiconductor lasers include a distributed Bragg reflection type semiconductor laser. The DFB semiconductor laser is used for a light source in a pickup device mounted on an optical apparatus for recording and reproducing reading out and writing information signals on an optical disc. The DFB lasers are also used in the fields of optical communication systems such as optical CATVs, optical measurement and the like.
The DFB semiconductor laser requires a grating having a periodic structure adjacent to the active region to improve the optical coupling. In the manufacture of the DFB semiconductor lasers, therefore, it requires to use two or more steps of epitaxial growth i.e., re-growth step. In the process of conventional re-growth step, a laser structure is grown by means of the metal organic chemical vapor deposition (MOCVD), then the grating is formed in the laser structure layer by etching, and thereafter another laser structure is formed on the grating again by the MOCVD.
Recently, in order to avoid such a complicated epitaxial re-growth, the so-called single-growth DFB semiconductor lasers have been suggested which are fabricated using one single step of epitaxial growth. For example, there is a gain coupled DFB semiconductor laser which has an asymmetrical cladding waveguide structure (M. L. Osowski, J. S. Hughes, R. M. Lammert, and J. J. Coleman xe2x80x9cAn Asymmetric Cladding Gain-Coupled DFB Laser with Oxide Defined Metal Surface Grating by MOCVDxe2x80x9d IEEE Photonics Technology Letters, Vol. 9, No. 11, November 1997 P1460-P1462).
As shown in FIG. 1, a gain coupled DFB semiconductor laser has an active layer 2a, an upper cladding layer 3 and a lower cladding layer 1 sandwiching the active layer 2, plurality of SiO2 insulators 6 formed on the upper cladding layer 3 with a cap stripe, and a metal electrode layer 7 of Tixe2x80x94Ptxe2x80x94Au completely covering the insulators and the cladding layer, in characterized in that the upper cladding layer 3 is formed thinner than the lower cladding layer 1 whereby the light intensity distribution becomes asymmetrical in a direction perpendicular to the longitudinal direction of waveguide. In such a ridge gain coupled DFB semiconductor laser, the excited light of laser leaks from the thinner upper cladding layer 3 in the grating region, because the thinner upper cladding layer 3 has poor absorption in the GaAs semiconductor laser. Therefore, the excited light of laser is moderated by the metal electrode layer 7 at the side of the thinner upper cladding layer 3 of top surface and laser-oscillated in a single axis mode.
When such a surface grating type gain coupled DFB semiconductor laser is manufactured with using InGaAs material, the property satisfied for the slope efficiency is not obtained.
The present invention confronts the above-described problem, and it is an object of the present invention to provide a laterally-coupled DFB semiconductor laser secure having complex coupling of index and gain couplings.
The object is achieved by a ridge type semiconductor laser of laterally-coupled distributed feedback in a first aspect of the invention comprises;
an active layer made of semiconductor;
a ridge stripe having a cladding layer formed on said active layer and a contact layer formed on the cladding layer to protrude from said active layer;
a pair of gratings each having a periodic structure in a longitudinal direction of the ridge stripe having a plurality of grooves each extending from side walls of the ridge stripe on flat portions in both sides of the ridge stripe; and
absorbing layers covering the surfaces of the grooves of gratings to absorb excited light.
In a second aspect of a semiconductor laser according to the invention, said absorbing layer comprises a first insulator kept contact with the surfaces of the grooves of gratings; a metal layer contiguously formed on the first insulator; and a second insulator contiguously formed on the metal layer.
In a third aspect of a semiconductor laser according to the invention, said absorbing layer is an insulator layer comprising an insulator material as a matrix and metal particles dispersed in the matrix.
In a fourth aspect of according to the invention, the semiconductor laser further comprises bracket grating portions each having a slope surface extending from a flat top portion of the ridge stripe to a top face of a land portion defined by the adjacent grooves and coupling the side walls of the ridge stripe and the gratings.
In a fifth aspect of a semiconductor laser according to the invention, said active layer is a bulk layer, a single quantum well layer, or a multiple quantum well layer mainly composed of In1xe2x88x92xGaxAs1xe2x88x92yPy (where 0xe2x89xa6x less than 1, 0xe2x89xa6yxe2x89xa61); and said cladding layer is made of InP.
In a sixth aspect of a semiconductor laser according to the invention, said contact layer is made of InGaAsP or InGaAs.
In a seventh aspect of a semiconductor laser according to the invention, the ridge stripe and the pair of gratings have a relationship between a waveguide without grating up and down and gratings laterally coupled thereto.